1. Field of the Inventions
The present inventions relate generally to a propulsion unit for a boat, and, more particularly, to controlling an idle speed control (“ISC”) valve of an engine of a boat.
2. Background of the Invention
Many different propulsion units, such as outboard motors, may be used to propel a boat. One such outboard motor is shown in Japanese Patent Application No. JP 2001-152895. In this application, the propulsion unit is supported by the boat's hull and includes a propeller. An internal combustion engine is also supported by the hull for driving the propeller of the propulsion unit.
The boat also includes a gear switching device that can change the state of the drive gears coupling the internal combustion engine and the propeller. Typically, these types of gear change transmissions allow an operator to switch between a forward drive mode, a reverse drive mode, and a neutral mode.
As is typical in the internal combustion engine art, a throttle valve controls the air flow through an intake passage that guides air from the atmosphere to the internal combustion engine. A secondary valve, also known as an idle speed control valve (“ISC valve”), controls the air flow through a secondary air passage configured to guide air from the atmosphere to the intake passage at a point downstream of the throttle valve. Finally, a control unit may be provided for controlling the idle speed of the engine by adjusting the position of the ISC valve. The system disclosed in JP 2001-152895 is configured to bring the internal combustion engine to a predetermined rotational speed when the throttle valve is closed quickly.
During operation of such a boat, with the internal combustion engine running, the switching device is shifted to the desired drive position so as to transmit power from the engine to the propeller. The throttle valve may also be controlled to adjust the opening of the intake passage, so that a desired amount of air can be introduced into the internal combustion engine. The amount of air introduced is generally proportional to the power produced by the engine. Thus, the operator can control both the direction and speed of the boat.
When the boat is cruising at a cruising speed and the throttle valve is closed quickly to decelerate the boat, the internal combustion engine is also rapidly decelerated, and the amount of intake air needed is decreased. However, in this typical deceleration scenario, the amount of intake air being supplied to the internal combustion engine may still be insufficient, even for the engine's reduced needs, and the engine can stall.
As discussed in the above referenced Japanese application, the control unit for the ISC valve preferably automatically opens the secondary air passage to bring the internal combustion engine to a target rotational speed. This secondary air prevents the internal combustion engine from stalling regardless of how quickly the throttle valve is closed. The internal combustion engine can thereby maintain a predetermined target rotational speed.